The present invention relates generally to the field of modems, and, more particularly, to modem startup protocols.
The demand for remote access to information sources and data retrieval, as evidenced by the success of services such as the World Wide Web, is a driving force for high-speed network access technologies. Today""s telephone network offers standard voice services over a 4 kHz bandwidth. Traditional analog modem standards generally assume that both ends of a modem communication session have an analog connection to the public switched telephone network (PSTN). Because data signals are typically converted from digital to analog when transmitted towards the PSTN and then from analog to digital when received from the PSTN, data rates may be limited to 33.6 kbps as defined in the V.34 transmission recommendation developed by the International Telecommunications Union (ITU).
The need for an analog modem can be eliminated, however, by using the basic rate interface (BRI) of the Integrated Services Digital Network (ISDN). A BRI offers end-to-end digital connectivity at an aggregate data rate of 160 kbps, which is comprised of two 64 kbps B channels, a 16 kbps D channel, and a separate maintenance channel. ISDN offers comfortable data rates for Internet access, telecommuting, remote education services, and some forms of video conferencing. ISDN deployment, however, has been very slow due to the substantial investment required of network providers for new equipment. Because ISDN is not very pervasive in the PSTN, the network providers have typically tarriffed ISDN services at relatively high rates, which may be ultimately passed on to the ISDN subscribers. In addition to the high service costs, subscribers must generally purchase or lease network termination equipment to access the ISDN.
While most subscribers do not enjoy end-to-end digital connectivity through the PSTN, the PSTN is nevertheless mostly digital. Typically, the only analog portion of the PSTN is the phone line or local loop that connects a subscriber or client modem (e.g., an individual subscriber in a home, office, or hotel) to the telephone company""s central office (CO). In recent years, local telephone companies have been replacing portions of their original analog networks with digital switching equipment. Nevertheless, the connection between the home and the CO has been the slowest to change to digital as discussed in the foregoing with respect to ISDN BRI service. A recent data transmission recommendation issued by the ITU, known as V.90, takes advantage of the digital conversions that have been made in the PSTN. By viewing the PSTN as a digital network, V.90 technology is able to accelerate data downstream from the Internet or other information source to a subscriber""s computer at data rates of up to 56 kbps, even when the subscriber is connected to the PSTN via an analog local loop.
To understand how the V.90 recommendation achieves this higher data rate, it may be helpful to briefly review the operation of V.34 analog modems. V.34 modems are optimized for the situation where both ends of a communication session are connected to the PSTN by analog lines. Even though most of the PSTN is digital, V.34 modems treat the network as if it were entirely analog. Moreover, the V.34 recommendation assumes that both ends of the communication session suffer impairment due to quantization noise introduced by analog-to-digital converters. That is, the analog signals transmitted from the V.34 modems are sampled at 8000 times per second by a codec upon reaching the PSTN with each sample being represented or quantized by an eight-bit pulse code modulation (PCM) codeword. The codec uses 256, non-uniformly spaced, PCM quantization levels defined according to either the xcexc-law or A-law companding standard (ie., the ITU G.711 Recommendation).
Because the analog waveforms are continuous and the binary PCM codewords are discrete, the digits that are sent across the PSTN can only approximate the original analog waveform. The difference between the original analog waveform and the reconstructed quantized waveform is called quantization noise, which limits the modem data rate.
While quantization noise may limit a V.34 communication session to 33.6 kbps, it nevertheless affects only analog-to-digital conversions. The V.90 standard relies on the lack of analog-to-digital conversions outside of the conversion made at the subscriber""s modem to enable transmission at 56 kbps.
The general environment for which the V.90 standard was developed is depicted in FIG. 1. An Internet Service Provider (ISP) 22 is connected to a subscriber""s computer 24 via a V.90 digital server modem 26, through the PSTN 28 via digital trunks (e.g., T1, E1, or ISDN Primary Rate Interface (PRI) connections), through a central office switch 32, and finally through an analog loop to the client""s modem 34. The central office switch 32 is drawn outside of the PSTN 28 to better illustrate the connection of the subscriber""s computer 24 and modem 34 into the PSTN 28. It should be understood that the central office 32 is, in fact, a part of the PSTN 28. The operation of a communication session between the subscriber 24 and an ISP 22 is best described with reference to the more detailed block diagram of FIG. 2.
Transmission from the server modem 36 to the client modem 34 will be described first. The information to be transmitted is first encoded using only the 256 PCM codewords used by the digital switching and transmission equipment in the PSTN 28. These PCM codewords are transmitted towards the PSTN 28 by the PCM transmitter 36 where they are received by a network code. The PCM data is then transmitted though the PSTN 28 until reaching the central office 32 to which the client modem 34 is connected. Before transmitting the PCM data to the client modem 34, the data is converted from its current form as either xcexc-law or A-law companded PCM codewords to pulse amplitude modulated (PAM) voltages by the codec expander (digital-to-analog (D/A) converter) 38.
These PAM voltage levels are processed by a central office hybrid 42 where the unidirectional signal received from the codec expander 38 is transmitted towards the client modem 34 as part of a bidirectional signal. A second hybrid 44 at the subscriber""s analog telephone connection converts the bidirectional signal back into a pair of unidirectional signals. Finally, the analog signal from the hybrid 44 is converted into digital PAM samples by an analog-to-digital (AID) converter 46, which are received and decoded by the PAM receiver 48. Note that for transmission to succeed effectively at 56 kbps, there must be only a single digital-to-analog conversion and subsequent analog-to-digital conversion between the server modem 26 and the client modem 34. Recall that analog-to-digital conversions in the PSTN 28 can introduce quantization noise, which may limit the data rate as discussed previously. The A/D converter 46 at the client modem 34, however, may have a higher resolution than the A/D converters used in the analog portion of the PSTN 28 (e.g., 16 bits versus 8 bits), which results in less quantization noise. Moreover, the PAM receiver 48 needs to be in synchronization with the 8 kHz network clock to properly decode the digital PAM samples.
Transmission from the client modem 34 to the server modem 26 follows the V.34 data transmission standard. That is, the client modem 34 includes a V.34 transmitter 52 and a D/A converter 54 that encode and modulate the digital data to be sent using techniques such as quadrature amplitude modulation (QAM). The hybrid 44 converts the unidirectional signal from the digital-to-analog converter 54 into a bidirectional signal that is transmitted to the central office 32. Once the signal is received at the central office 32, the central office hybrid 42 converts the bidirectional signal into a unidirectional signal that is provided to the central office codec. This unidirectional, analog signal is converted into either xcexc-law or A-law companded PCM codewords by the codec compressor (A/D converter) 56, which are then transmitted through the PSTN 28 until reaching the server modem 26. The server modem 26 includes a conventional V.34 receiver 58 for demodulating and decoding the data sent by the V.34 transmitter 52 in the client modem 34. Thus, data is transferred from the client modem 34 to the server modem 26 at data rates of up to 33.6 kbps as provided for in the V.34 standard.
The V.90 standard only offers increased data rates (e.g., data rates up to 56 kbps) in the downstream direction from a server to a subscriber or client. Upstream communication still takes place at conventional data rates as provided for in the V.34 standard. Nevertheless, this asymmetry is particularly well suited for Internet access. For example, when accessing the Internet, high bandwidth is most useful when downloading large text, video, and audio files to a subscriber""s computer. Using V.90, these data transfers can be made at up to 56 kbps. On the other hand, traffic flow from the subscriber to an ISP consists of mainly keystroke and mouse commands, which are readily handled by the conventional rates provided by the V.34 standard.
The V.90 standard, therefore, provides a framework for transmitting data at rates up to 56 kbps provided the network is capable of supporting the higher rates. The most notable requirement is that there can be at most one digital-to-analog conversion and no analog-to-digital conversion in the downstream path within the network. Nevertheless, other digital impairments, such as robbed bit signaling (RBS) and digital mapping through PADs which results in attenuated signals, can also inhibit transmission at V.90 rates. Communication channels exhibiting non-linear frequency response characteristics are yet another impediment to transmission at the V.90 rates. Moreover, these other factors may limit conventional V.90 performance to less than the 56 kbps theoretical data rate.
Articles such as Humblet et al., xe2x80x9cThe Information Driveway,xe2x80x9d IEEE Communications Magazine, December 1996, pp. 64-68, Kalet et al., xe2x80x9cThe Capacity of PCM Voiceband Channels,xe2x80x9d IEEE International Conference on Communications ""93, May 23-26, 1993, Geneva, Switzerland, pp. 507-511, Fischer et al., xe2x80x9cSignal Mapping for PCM Modems,xe2x80x9d V-pcm Rapporteur Meeting, Sunriver, Oregon, USA, Sep. 4-12, 1997, and Proakis, xe2x80x9cDigital Signaling Over a Channel with Intersymbol Interference,xe2x80x9d Digital Communications, McGraw-Hill Book Company, 1983, pp. 373, 381, provide general background information on digital communication systems.
One problem encountered by modems communicating over channels having topologies which include digital to analog (D/A) conversions is that an echo may be generated by the associated two to four wire conversion interface. Server modems typically are provided with echo canceling circuitry to compensate for the expected echo which results from the D/A conversion required for the final leg of the connection over the analog local loop even in network topologies that are otherwise all digital. This echo is sometimes referred to as near end echo generated by the circuitry of the server modem""s own network. However, some network topologies include one or more extra DI/A conversions in the channel which results in a condition referred to as a digital discontinuity. In addition to causing a V.90 modem to fall back to V.34 mode, a digital discontinuity located on the local loop to a client modem may also generate an additional echo which may be referred to as a middle echo as it typically has a delay longer than the near end echo. An example of a network topology which may generate such a middle echo is one which includes a Universal Digital Loop Carrier (UDLC) system in the local loop between the client modem and the server modem. Such middle echos may not be effectively canceled by the echo canceling circuit of prior art modems which are typically designed to control near end echo. Accordingly, a need exists for improved systems and methods for modem communications.
It is an object of the present invention to provide methods, systems, and computer program products which may be able to improve performance of a modem communication session over a channel subject to echo noise.
These and other objects, advantages, and features of the present invention are provided by methods, systems, and computer program products for configuring a modem communication session when conditions associated with echo noise on a digitally discontinuous channel are detected. The condition is detected at the local modem by reference to the local modem""s respective transmit and receive rates. A low transmit rate is an indication of a channel problem. Furthermore, where the transmit rate is also lower than the receive rate, the problem is likely at the remote modem end not just a uniformly noisy channel. This combination of conditions has been found to result, for example, from middle echo noise. Once the remote modem side condition is detected various steps may be taken by the local modem to improve the channel performance including boosting its transmit power (which may raise signal power at the remote modem receiver without affecting the level of echo noise), enabling only a low symbol rate for its transmissions (which may concentrate the signal power in a narrower spectrum band to improve signal power) and/or disabling the use of strong pre-emphasis by the remote modem (which may reduce echo noise by reducing remote modem transmit power over a portion of the spectrum). In particular, these approaches have been found to improve performance of V.90 modems operating in fall back (V.34 ) mode.
In one embodiment of the present invention, a method is provided for configuring a V.34 capable modem for a communication session with a remote modem. A receive data rate from the remote modem and a transmit data rate to the remote modem are detected during startup of the communication session. A retrain is then initiated when the transmit data rate is less than the receive data rate if the transmit data rate is less than a predetermined criterion. Transmit power to the remote modem of a transmitter of the modem is boosted during the retrain. In one embodiment, the communication session is on a digitally discontinuous channel subject to echo noise and the transmit power is boosted to provide a higher signal to echo noise ratio at the remote modem.
In another embodiment of the present invention, the retrain is initiated after setting a digital discontinuity flag which is set when the transmit data rate is less than the receive data rate if the transmit data rate is less than the predetermined criterion. The transmit power is boosted during the retrain when the digital discontinuity flag is set.
In a further embodiment of the present invention, a predetermined symbol rate is selected for the transmitter when the transmit data rate is less than the receive data rate if the transmit data rate is less than the predetermined criterion. In a further aspect, strong pre-emphasis for a transmitter of the remote modem is disabled when the transmit data rate is less than the receive data rate if the transmit data rate is less than the predetermined criterion. The modem and the remote modem in one embodiment are either a V.34 standard modem or a V.90 standard modem, the predetermined symbol rate may be 2400 symbols per second and pre-emphasis may be disabled for pre-emphasis indexes of 6 and above. In a further embodiment, initiating retrain operations include setting a digital discontinuity flag when the transmit data rate is less than the receive data rate if the transmit data rate is less than the predetermined criterion. A predetermined symbol rate is selected for the transmitter when the digital discontinuity flag is set. Strong pre-emphasis for a transmitter of the remote modem is disabled when the discontinuity flag is set. The selecting and disabling operations may be performed during phase 2 of startup of the communication session and the detecting and initiating operations may be performed during phase 4 of startup of the communication session. The 2400 symbol rate may be specified by specifying 2400 symbols per second as the symbol rate in the INFO1a signal.
As will further be appreciated by those of skill in the art, while described above primarily with reference to method aspects, the present invention may be embodied as methods, apparatus/systems, and/or computer program products.